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![LangChain](https://pica.zhimg.com/50/v2-56e8bbb52aa271012541c1fe1ceb11a2_r.gif)





检索问答[#](#retrieval-question-answering "标题的永久链接")
================================================

这个示例展示了如何在索引上进行问答。

```python
from langchain.embeddings.openai import OpenAIEmbeddings
from langchain.vectorstores import Chroma
from langchain.text_splitter import CharacterTextSplitter
from langchain.llms import OpenAI
from langchain.chains import RetrievalQA

```

```python
from langchain.document_loaders import TextLoader
loader = TextLoader("../../state_of_the_union.txt")
documents = loader.load()
text_splitter = CharacterTextSplitter(chunk_size=1000, chunk_overlap=0)
texts = text_splitter.split_documents(documents)

embeddings = OpenAIEmbeddings()
docsearch = Chroma.from_documents(texts, embeddings)

```

```python
Running Chroma using direct local API.
Using DuckDB in-memory for database. Data will be transient.

```

```python
qa = RetrievalQA.from_chain_type(llm=OpenAI(), chain_type="stuff", retriever=docsearch.as_retriever())

```

```python
query = "What did the president say about Ketanji Brown Jackson"
qa.run(query)

```

```python
" The president said that she is one of the nation's top legal minds, a former top litigator in private practice, a former federal public defender, and from a family of public school educators and police officers. He also said that she is a consensus builder and has received a broad range of support, from the Fraternal Order of Police to former judges appointed by Democrats and Republicans."

```

链类型[#](#chain-type "标题的永久链接")
-----------------------------

你可以轻松地指定不同的链类型来加载和使用RetrievalQA链。有关这些类型的更详细的演示，请参见[这个notebook](question_answering)。

有两种加载不同链类型的方法。首先，你可以在`from_chain_type`方法中指定链类型参数。这允许你传入你想要使用的链类型的名称。例如，在下面的例子中，我们将链类型更改为`map_reduce`。

```python
qa = RetrievalQA.from_chain_type(llm=OpenAI(), chain_type="map_reduce", retriever=docsearch.as_retriever())

```

```python
query = "What did the president say about Ketanji Brown Jackson"
qa.run(query)

```

```python
" The president said that Judge Ketanji Brown Jackson is one of our nation's top legal minds, a former top litigator in private practice and a former federal public defender, from a family of public school educators and police officers, a consensus builder and has received a broad range of support from the Fraternal Order of Police to former judges appointed by Democrats and Republicans."

```

以上方法允许你非常简单地更改链类型，但它确实提供了对该链类型参数的许多灵活性。如果你想控制这些参数，你可以直接加载链（就像在[这个notebook](question_answering)中所做的那样)，然后将其直接传递给RetrievalQA链的`combine_documents_chain`参数。例如：

```python
from langchain.chains.question_answering import load_qa_chain
qa_chain = load_qa_chain(OpenAI(temperature=0), chain_type="stuff")
qa = RetrievalQA(combine_documents_chain=qa_chain, retriever=docsearch.as_retriever())

```

```python
query = "What did the president say about Ketanji Brown Jackson"
qa.run(query)

```

```python
" The president said that Ketanji Brown Jackson is one of the nation's top legal minds, a former top litigator in private practice, a former federal public defender, and from a family of public school educators and police officers. He also said that she is a consensus builder and has received a broad range of support from the Fraternal Order of Police to former judges appointed by Democrats and Republicans."

```

自定义提示[#](#custom-prompts "Permalink to this headline")
------------------------------------------------------

您可以传递自定义提示来进行问答。这些提示与您可以传递到[基础问答链](question_answering)中的提示相同。

```python
from langchain.prompts import PromptTemplate
prompt_template = """Use the following pieces of context to answer the question at the end. If you don't know the answer, just say that you don't know, don't try to make up an answer.

{context}

Question: {question}
Answer in Italian:"""
PROMPT = PromptTemplate(
    template=prompt_template, input_variables=["context", "question"]
)

```

```python
chain_type_kwargs = {"prompt": PROMPT}
qa = RetrievalQA.from_chain_type(llm=OpenAI(), chain_type="stuff", retriever=docsearch.as_retriever(), chain_type_kwargs=chain_type_kwargs)

```

```python
query = "What did the president say about Ketanji Brown Jackson"
qa.run(query)

```

```python
" Il presidente ha detto che Ketanji Brown Jackson è una delle menti legali più importanti del paese, che continuerà l'eccellenza di Justice Breyer e che ha ricevuto un ampio sostegno, da Fraternal Order of Police a ex giudici nominati da democratici e repubblicani."

```

返回源文档[#](#return-source-documents "Permalink to this headline")
---------------------------------------------------------------

此外，我们可以在构建链时指定一个可选参数来返回用于回答问题的源文档。

```python
qa = RetrievalQA.from_chain_type(llm=OpenAI(), chain_type="stuff", retriever=docsearch.as_retriever(), return_source_documents=True)

```

```python
query = "What did the president say about Ketanji Brown Jackson"
result = qa({"query": query})

```

```python
result["result"]

```

```python
" The president said that Ketanji Brown Jackson is one of the nation's top legal minds, a former top litigator in private practice and a former federal public defender from a family of public school educators and police officers, and that she has received a broad range of support from the Fraternal Order of Police to former judges appointed by Democrats and Republicans."

```

```python
result["source_documents"]

```

```python
[Document(page_content='Tonight. I call on the Senate to: Pass the Freedom to Vote Act. Pass the John Lewis Voting Rights Act. And while you’re at it, pass the Disclose Act so Americans can know who is funding our elections.   Tonight, I’d like to honor someone who has dedicated his life to serve this country: Justice Stephen Breyer—an Army veteran, Constitutional scholar, and retiring Justice of the United States Supreme Court. Justice Breyer, thank you for your service.   One of the most serious constitutional responsibilities a President has is nominating someone to serve on the United States Supreme Court.   And I did that 4 days ago, when I nominated Circuit Court of Appeals Judge Ketanji Brown Jackson. One of our nation’s top legal minds, who will continue Justice Breyer’s legacy of excellence.', lookup_str='', metadata={'source': '../../state_of_the_union.txt'}, lookup_index=0),
 Document(page_content='A former top litigator in private practice. A former federal public defender. And from a family of public school educators and police officers. A consensus builder. Since she’s been nominated, she’s received a broad range of support—from the Fraternal Order of Police to former judges appointed by Democrats and Republicans.   And if we are to advance liberty and justice, we need to secure the Border and fix the immigration system.   We can do both. At our border, we’ve installed new technology like cutting-edge scanners to better detect drug smuggling.    We’ve set up joint patrols with Mexico and Guatemala to catch more human traffickers.    We’re putting in place dedicated immigration judges so families fleeing persecution and violence can have their cases heard faster.   We’re securing commitments and supporting partners in South and Central America to host more refugees and secure their own borders.', lookup_str='', metadata={'source': '../../state_of_the_union.txt'}, lookup_index=0),
 Document(page_content='And for our LGBTQ+ Americans, let’s finally get the bipartisan Equality Act to my desk. The onslaught of state laws targeting transgender Americans and their families is wrong.   As I said last year, especially to our younger transgender Americans, I will always have your back as your President, so you can be yourself and reach your God-given potential.   While it often appears that we never agree, that isn’t true. I signed 80 bipartisan bills into law last year. From preventing government shutdowns to protecting Asian-Americans from still-too-common hate crimes to reforming military justice.   And soon, we’ll strengthen the Violence Against Women Act that I first wrote three decades ago. It is important for us to show the nation that we can come together and do big things.   So tonight I’m offering a Unity Agenda for the Nation. Four big things we can do together.    First, beat the opioid epidemic.', lookup_str='', metadata={'source': '../../state_of_the_union.txt'}, lookup_index=0),
 Document(page_content='Tonight, I’m announcing a crackdown on these companies overcharging American businesses and consumers.   And as Wall Street firms take over more nursing homes, quality in those homes has gone down and costs have gone up.    That ends on my watch.   Medicare is going to set higher standards for nursing homes and make sure your loved ones get the care they deserve and expect.   We’ll also cut costs and keep the economy going strong by giving workers a fair shot, provide more training and apprenticeships, hire them based on their skills not degrees.   Let’s pass the Paycheck Fairness Act and paid leave.    Raise the minimum wage to $15 an hour and extend the Child Tax Credit, so no one has to raise a family in poverty.   Let’s increase Pell Grants and increase our historic support of HBCUs, and invest in what Jill—our First Lady who teaches full-time—calls America’s best-kept secret: community colleges.', lookup_str='', metadata={'source': '../../state_of_the_union.txt'}, lookup_index=0)]

```

